The present invention generally relates to phase locked pulse generators apparatus and, more particularly, to such generators that are adapted to generate pulses synchronized to digital data streams.
The continued advances in technology have resulted in many changes in the equipment that is currently being used in television broadcast stations. One of the more recent changes that has evolved is the shift away from photographic techniques toward the use of magnetic media in many phases of the operation of the commercial broadcast television station. For example, feature films being broadcast often originate from magnetic tape rather than film and television station news departments are increasingly converting to videotape recording systems rather than using film cameras to provide the visual coverage of the news stories. Moreover, many systems utilize travelling transmitters that can either broadcast on location coverage or transmit such coverage to the station which can either be broadcast "live" or videotaped, edited and broadcast at a later time. Some of the many benefits of these techniques are the ease of handling, flexibility and speed of processing compared to the use of photographic film, coupled with the ability to reuse the magnetic tape when the information that is recorded on them is no longer needed.
One of the last remaining film domains in the present day commercial television broadcasting station is the Telecine island which uses 35 millimeter film transparencies. The Telecine island is used to provide video still images that are used during programming, commercials, news and the like, i.e., wherever a still image may be used during operation. Their use is extensive as is evidenced by the fact that the average commercial broadcast television station maintains a total file on the order of about 2000 to 5000 35 millimeter transparency slides. The maintenance of the total file represents a laborious operation which requires introduction of new slides, the discarding of obsolete slides and the maintenance of an accurate index so that they can be readily obtained when needed. When slide program sequences are to be assembled, they must be manually carried to the Telecine island, cleaned and manually loaded. Even with the cleaning operation, dust particles and scratches and the like may easily result in an unsatisfactory end product even when the projectionist is careful. Moreover, following their use during broadcasting, the slides must be removed and returned to the file. The entire assembling, use and refiling of the slides represents a substantial labor investment because of the many manual operations that are required. The Telecine operation is considered to be one of the most antiquated operations in many modern broadcast stations and is basically incompatible with a fully automated station operation.
In contrast to the Telecine island or the use of opaque graphic material as the source for generating video still images, the present invention facilitates the use of a recording and playback apparatus that will record and reproduce still images, with the still image video information being stored on magnetic media. The magnetic recording and playback apparatus utilizing the present invention facilitates the use of a self clocking digital data code to transmit digital data between distant points and, hence, the use of standard computer disc drives (though modified in some respects) as the magnetic storage media and thereby eliminates the many problems that are associated with slide transparencies. Since the still images are recorded on magnetic media, the problems of physical degradation during use, e.g., dust particles and scratches, are not experienced. Moreover, since the recorded information can be easily accessed, the same still image may be used by operators at different locations almost simultaneously.
The present invention is a phase locked clock generator for use in decoding a digital data stream of the kind in which the data content is determined by a sequence of signal state changes, either in the form of narrow data pulses or signal level transitions, each change being located in one of two positions within a data cell interval. The phase locked clock generator is useful for decoding digital data streams in which the signal state changes are located at either an edge position or a midpoint position of the data cell interval. The exemplary embodiment of the phase locked clock generator is disclosed herein as used in a decoder arranged to decode digital data streams encoded according to the code rules described in either U.S. Pat. No. 3,108,261 or No. 4,027,335. As will be described hereinbelow, digital data streams encoded according to each of the code rules described in those patents have signal state changes located at either the edge or midpoint positions of the data cells. The phase locked clock generator is arranged to provide clock pulses at a nominal rate of two times the encoded data clock rate so that, when properly phase locked, a clock pulse will be provided synchronously with the edge and midpoint positions of each data cell of the encoded digital data stream. For phase locking the clock generator, a narrow data pulse corresponding to each signal state change is provided to a phase comparator for comparison with the clock pulses. The phase comparator includes a current source that responds to the data and clock pulses to provide current to a charging circuit for an interval determined by selected edge transitions of the pulses received by the comparator. The charging circuit is coupled to the clock generator so that the generator is servoed to provide clock pulses to the phase comparator with its selected edge transition located at the midpoint of the narrow data pulse. In the absence of data pulses, the current source is controlled to provide current to the charging circuit that maintains the clock signal provided by the clock signal generator at a nominal frequency and phase.
While the phase locked clock pulse generator of the present invention is suited for use in apparatus for recording and reproducing still images and a preferred embodiment is described herein as arranged for such use, the phase locked clock pulse generator can be arranged for other uses wherein digital data in a self clocking code form is required to be decoded.